Test Device for Verifying Operation of an Optical Fiber Monitoring System Protecting Multiple Fiber Runs

ABSTRACT

In an optical fiber monitoring system which detects physical disturbance or other parameters such as temperature or strain of a fiber where a monitor signal is transmitted along the optical fiber and analyzed to detect changes which are indicative of an event, a method is provided for periodically checking proper operation of the optical fiber monitoring system. A fiber disturbance actuator periodically causes a pattern of disturbances of a portion of the fiber at a predetermined location thereon where the disturbance is characteristic of the event to be monitored. The monitor signal is analyzed to detect the pattern of changes and in the event that expected changes are not detected, a warning is issued that the intrusion detection system is not properly operating.

This application is a continuation in part application of applicationSer. No. 17/018,535 filed Sep. 11, 2020.

This application relates to a test device for periodically, or ondemand, checking and verifying proper operation of a monitoring systemof an optical fiber.

BACKGROUND OF THE INVENTION

Optical fiber is used for many types of monitoring applications,including but not limited to perimeter security, network security,structural monitoring.

Typically the fiber concerned is monitored using a method which includestransmitting from a source of light at a transmit location a monitorsignal along the optical fiber, receiving the monitor signal aftertransmission along the fiber, analyzing the monitor signal aftertransmission along the fiber to detect changes therein and generating analarm in response to the detected changes.

In regard to communications networks, the monitor system is responsiveto vibration, motion, or handling of the fiber which are indicative ofan intrusion attempt on the fiber.

In addition, the invention herein can be used for fence and buriedperimeter protection systems where a fiber is mounted on or at the itemto be secured so that again the fiber is monitored for vibration ormotion of the fiber caused by attempts to access or penetrate the itemconcerned.

Yet further, the invention herein can be used for other fibers used formonitoring forces on the fiber caused by strain or other forces thatmonitor bridge or building integrity. These can include stretching orcompression of the fiber. In this case the monitor Is not looking fortransverse vibration or movement of the fiber from an intrusion attemptor other handling but is instead looking for changes in the character ofthe fiber caused by the application of the forces to the fiber. Sucharrangement can be used in strain gauges, building and bridge monitoringsystems and the like.

Additionally, the invention herein can be used to monitor temperature,strain, and pressure using sensors in wells and down-hole applications.Such methods require a looped fiber to accommodate the inability toplace equipment in wells or down holes.

Additionally, the invention herein can be used to monitor fibersdistributed throughout a so-called ‘smart city” type application. Inthese instances, fibers are distributed to monitor traffic patterns,weather, electrical distribution, and seismic activity.

In all cases the change in the parameter to be measured causes a changein a characteristic in the fiber which can be measured using knowntechniques.

One method for monitoring a communications network cable is to usefibers that are internal to the protected cable. This so-called“intrinsic monitoring” is shown in U.S. Pat. No. 7,706,641 issued Apr.27 2010 to the present applicant, the disclosure of which isincorporated herein by reference.

The optical fibers can be monitored using a variety of detectiontechniques including:

Modal metric, where changes in a modal power distribution in a multimodefiber are detected as shown in U.S. Pat. No. 7,092,586 issued Aug. 152006 to the present applicant, the disclosure of which is incorporatedherein by reference.

Attenuation, where simply an attenuation in the monitoring signalreceived is measured.

Optical Time Domain Reflectometer (OTDR) where reflections or localizedattenuations from components of the fiber are detected.

Distributed sensing (DAS/DSS/DTS):

DAS—Distributed Acoustic Sensing where vibrations and displacementscause localized shifts in the path length of the optical fiber. This isdetected by a high precision optical Time Domain Reflectometer (OTDR).This OTDR is often referred to as a Phase-OTDR or Φ-OTDR, and measureschanges in the distance between points of Rayleigh backscatter.

DSS—Distributed Strain Sensing—where strain is measured along a fiberdue to tensile or compressive displacements, compression, or cracks.Typically measured using Brillouin OTDR, transmitted light and scatteredlight are mixed as a heterodyne receiver. This Brillouin frequency shiftis proportional to strain and temperature in the fiber.

DTS—Distributed Temperature Sensor—where temperature is measured alongan optical fiber including by use of Raman OTDR. Light propagating downthe fiber at two wavelengths cause Stokes and anti-Stokes light. Theamplitude of light reflected back to the detector in a similar fashionto Rayleigh Backscattering in a traditional OTDR, is highly dependent ontemperature. The ratio of Stokes and anti-Stokes light indicatestemperature, while the round trip transit time indicates location. Aswith DSS, Brillouin OTDR can also be used to measure distributedtemperature.

Polarization monitoring, where changes in a polarization in the signalin a single mode fiber are detected as shown in U.S. Pat. No. 7,142,737issued Nov. 28 2006 to the present applicant, the disclosure of which isincorporated herein by reference.

Active fiber monitoring, where monitoring signal and data signal pass onthe same fibers as shown in U.S. Pat. No. 7,092,586 issued Aug. 15 2006to Vokey et al. and the present applicant, the disclosure of which isincorporated herein by reference.

Strain monitoring such as strain gauge where a Fiber Bragg Grating,strain gauge or DSS monitors a fiber or mechanical structure,disturbance will be stretching or compression

Interferometry such as the Mach-Zehnder interferometers used for networkand perimeter monitoring. These may be zone based, or locating by use ofbidirectional differential time of flight systems.

Each of these methods of monitoring exploit a specific attribute of thefiber—be it loss, rotation of state of polarization, Rayleighscattering, or others.

In some cases a single fiber is monitored with typically thetransmission at one end and the monitoring at the other or same end.However other arrangements can be used in the present inventionincluding for example the loop type network shown for example in U.S.Pat. No. 7,142,737 issued Nov. 28 2006 to the present applicant, thedisclosure of which is incorporated herein by reference

Often these systems are placed into service, but continued viability andavailability should be verified. Accordingly, an intelligent, automatictest is needed that does not interfere with or disrupt the actualperformance of the monitoring fiber. The test should exercise the entiresystem, including perturbation detection, alarm generation, response toalarm, and alarm logging. Thus, it is desirable to create a controlledand characterized event on the monitor fiber structure itself to testthe entire system and responses.

One existing arrangement for this function is provided by the StopLight(trademark) available from CyberSecure IMS. This test is implemented bypassing the monitoring fiber in a secure network through an opticalshutter that simply shuts off transmission of the light to the fiber. Atthe appropriate time, scheduled, spontaneous, or random, the opticalshutter opens the circuit, causing the monitoring equipment to registeran alarm. This method is accepted in the industry, although ashortcoming is that the monitoring is “blind” during this short periodof testing. This adds the vulnerability of an inside threat, cognizantof the blind period, which could act to switch the monitored fibers witha substitute fiber or fiber pair. The system owners would then operateunder the belief that the system was secure, although in fact it is nolonger monitored.

SUMMARY OF THE INVENTION

According to one feature of the present invention there is provided amethod for verifying operation of an optical fiber monitoring systemcomprising:

-   -   using a detection system to detect changes in an optical fiber        caused by an event to be monitored by:        -   transmitting from a source of light at a transmit location a            monitor signal along the optical fiber;        -   receiving the monitor signal after transmission along the            fiber;        -   analyzing the monitor signal after transmission along the            fiber to detect changes therein caused by the event to be            monitored;        -   and generating an alarm in response to said detected changes            which are indicative of an event;    -   and periodically checking proper operation of the optical fiber        monitoring system by:        -   providing a fiber disturbance actuator to cause disturbance            of a portion of the fiber at a predetermined location            thereon where the disturbance is characteristic of an event            to be monitored;        -   periodically operating the fiber disturbance actuator;        -   analyzing the monitor signal to detect changes therein            caused by said fiber disturbance actuator;        -   and in the event that expected changes in response to said            fiber disturbance actuator are not detected, actuating a            warning that the intrusion detection system is not properly            operating.

The fiber concerned can be used for transmission of data as a singletransmission fiber or as part of a network. Alternatively, the fiber maybe used for monitoring movement of or damage to the fiber as part of aperimeter or structure monitoring system where the signal is used todetect the movement or vibrations of the fiber as a part of an intrusionevent into the area being monitored.

Preferably the fiber disturbance actuator generates a predeterminedpattern of movements which can thus be recognized by a signatureassociated with the pattern.

Preferably the fiber disturbance actuator generates predeterminedperiodic displacements to the fiber typically at a predeterminedfrequency. These displacements can be formed in bursts of a series ofspaced envelopes each containing predetermined periodic displacements tothe fiber.

Preferably the fiber disturbance actuator generates a modulationfrequency which is chosen to be out of band with ambient disturbances.

Preferably the fiber disturbance actuator generates a modulationfrequency which provides a duty cycle defining the frequency burstswhich is chosen to be dissimilar to natural occurrences.

Preferably the method includes analyzing the monitor signal andadjusting a magnitude of disturbance caused by the fiber disturbanceactuator in response to the analysis. This will allow the amplitude ofthe perturbation to be adjusted to be appropriate to the magnitude ofthe monitor signal so that the amplitude must be high enough to bedetected, but not so high as to interfere with the monitor signalprocessing. The adjustment can be controlled from any of severallocations, including over a network from the monitor end of the fiber,or controlled locally such as a local network, manual adjustment, orserial connection.

The amplitude of the test perturbation can be monitored to detect fiberquality as insertion loss changes or the insertion of a fiber optic tapalong the fiber.

The arrangement herein thus provides a system for applying a knowndisturbance or perturbation on a monitor fiber with the intention ofbeing detected by the monitoring system.

This invention excites the fiber under very specific conditions in orderto confirm that the fiber, cable, or structure is being monitored asexpected.

In a preferred arrangement, the fiber being monitored includes or hasconnected as a part an input fiber length for isolation from themonitoring device of any connector reflections caused by connection ofthe fiber disturbance actuator. In many cases this includes also an exitfiber which again acts for isolation purposes. This isolation providedby the inlet and exit fiber portions is particularly important in thesystems which rely on reflection techniques where any such reflectionscan be orders of magnitude greater than the signals to be detected.

This system including the inlet and exit isolation fiber portions istypically connected to a network and is instructed to commence a test.The internal fiber disturbance actuator is engaged in a pre-determinedmanner, the signal from which is then recognized by the monitoringdevice, which in turn registers the test as successful and proves thatthe fiber being monitored is connected and secure.

In a preferred embodiment, where the monitor equipment is of a type thatdetermines location of the movement rather than simply the existence ofthe movement, the monitoring system acts to look for a specific locationand for a specific signal signature. This will not only ensure theinvention is still connected to the monitored fiber, and by applyingdetection to a very specific location on the fiber it ensures that asubstitute fiber is not used to bypass the intended monitored cable.

This system satisfies a number of applications:

Under command from a communicating device such as a data network orserial port, the fiber disturbance actuator can be instructed toactivate. When this activation is detected by the monitoring device,satisfaction of periodic health check of the monitoring system issatisfied. This test may be in satisfaction of a formal standard, or asgood practice as defined by the user.

The fiber disturbance actuator may have a manual trigger such as a pushbutton, causing a test to initiate. This test might occur immediately orafter a predetermined or random time, adding confidence to the test.

The fiber disturbance actuator and the control system operating theactuator can in some cases be configured to initiate tests autonomously;where this might be scheduled or random in occurrence.

The fiber disturbance actuator may be used as a health monitor of thesecurity system. In this embodiment, the control system can initiate atest at a predetermined frequency, verifying the system is functional.

The arrangement can perform periodic connectivity test to confirm theintended fiber is being monitored.

As cited above in U.S. Pat. No. 7,706,641, fiber security products, suchas the Interceptor (trademark) product from Network Integrity Systemsperform a fiber monitoring function that is zone based. Rather thanpinpoint a location, these devices utilize a loop of fiber, and monitorthe entire continuous loop as one zone. The arrangement herein isapplicable for zone use as well. The fiber disturbance function of thefiber disturbance actuator acts similarly to a location determiningproduct.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided a method for verifyingoperation of an optical fiber monitoring system comprising:

-   -   wherein the optical fiber has a first end and a second end;    -   using a detection system to detect changes in an optical fiber        caused by an event to be monitored by:        -   at the first end of the optical fiber transmitting from a            source of light a monitor signal along the optical fiber;        -   at the second end returning the monitor signal along the            fiber to the first end;        -   at the first end of the optical fiber receiving the monitor            signal after transmission along the fiber;        -   analyzing the monitor signal after transmission along the            fiber to detect changes therein caused by the event to be            monitored;    -   and periodically checking proper operation of the optical fiber        monitoring system by:        -   providing an optical shutter at the second end of the fiber;        -   periodically operating the optical shutter to temporarily            terminate transmission of light along the fiber;        -   and analyzing the monitor signal to detect a termination            therein caused by said optical shutter.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided a method for operationof an optical fiber monitoring system comprising:

-   -   using a detection system to detect changes in an optical fiber        caused by one or more events on the fiber by:        -   receiving a monitor signal after transmission along the            fiber;        -   analyzing the monitor signal after transmission along the            fiber to detect changes therein caused by the event to be            monitored;        -   and generating an alarm in response to said detected changes            which are indicative of an event;    -   generating a waveform which is representative of one event of        said events;    -   at a predetermined location on the fiber, periodically operating        an actuator to cause changes in the signal to be analyzed;    -   the actuator being operated based on said waveform.

In this arrangement, one available option is for modifying the analysisbased on the received monitor signal caused by the changes so as to tunethe analysis.

In this arrangement, one available option is that the waveform isgenerated by recording an actual event.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided a method for verifyingoperation of an optical fiber monitoring system comprising:

-   -   using a detection system to detect changes in an optical fiber        caused by an event to be monitored by:        -   transmitting from a source of light at a transmit location a            monitor signal along the optical fiber;        -   receiving the monitor signal after transmission along the            fiber;        -   and analyzing the monitor signal after transmission along            the fiber to detect changes therein caused by the event to            be monitored;    -   and periodically checking proper operation of the optical fiber        monitoring system by:        -   at a predetermined location on the fiber periodically            operating an actuator to cause a disturbance which changes            the monitor signal in the fiber;        -   analyzing the monitor signal to detect changes therein            caused by said disturbance;        -   and in the event that expected changes in response to said            disturbance are not detected, actuating a warning that the            intrusion detection system is not properly operating;    -   wherein an instruction signal is communicated along the fiber to        the actuator to effect said operating of the actuator and there        is provided a coupler on the fiber to extract the instruction        signal from other signals in the fiber.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided a method for verifyingoperation of an optical fiber monitoring system:

-   -   wherein the system comprises a first fiber and a second fiber;    -   each of the first and second fibers extending from a transmit        location to a remote location and returning along a continuous        optical path to the transmit location;    -   each of the first and second fibers defining first and second        ends at the transmit location thus defining outward and return        portions of the fiber;    -   the method comprising:    -   using a detection system to detect changes in an optical fiber        caused by an event to be monitored by:    -   transmitting from at least one source of light at the transmit        location into the first and second ends of each of the first and        second fibers monitor signals so as to travel along the optical        fiber;    -   receiving the monitor signals after transmission along the        fibers;    -   analyzing the monitor signals after transmission along the fiber        to detect changes therein caused by disturbances on the fiber;    -   and periodically checking proper operation of the optical fiber        monitoring system by:        -   at a predetermined location on the first and second fibers            periodically operating an actuator to cause a disturbance            which changes the monitor signal in the fiber;        -   analyzing the monitor signal to detect changes therein            caused by disturbance;        -   and in the event that expected changes in response to said            disturbance are not detected, actuating a warning that the            intrusion detection system is not properly operating.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided a method for monitoringan optical fiber comprising:

-   -   transmitting from a source of light at a transmit location a        monitor signal along the optical fiber;    -   receiving the monitor signal after transmission along the fiber;    -   analyzing the monitor signal after transmission along the fiber        to detect changes therein caused by disturbances on the fiber;    -   and periodically generating disturbances in the fiber by        generating longitudinal forces in the fiber at one location        along the length relative to another location along the length        so as to cause changes in length of the fiber between the first        and second locations.

In one specific arrangement, the disturbances can be generated by alongitudinally moving anchor located between two stationary anchors.

In one specific arrangement, the disturbances can be generated by a bywrapping the fiber around a support to form two adjacent lengths andcommonly generating said longitudinal forces in said adjacent lengths.

In one embodiment for causing the longitudinal forces the supportcomprises a stationary anchor around which the fiber is wrapped andthere is provided a longitudinally moving anchor movable towards andaway from the stationary anchor. As detection systems monitorinterference within the fiber, and therefor may exhibit points ofconstructive and destructive interference (nulls), it can be desirableto excite more than a single point along the fiber. In FIG. 16, thefiber enters the apparatus, and is fixed to the stationary anchor. Thefiber then proceeds to the floating support to which it is fixed, thenaround the dowel which reverses direction while maintaining properminimum bend radius of the fiber, and again fixed to the floatingsupport and stationary anchor. By displacing the floating support by thedisturbance actuator, longitudinal strain is placed in 4 locations ofthe fiber—that between the anchor, support, and dowel; in bothdirections.

In another embodiment the longitudinal forces in the fiber at aregenerated by wrapping the fiber around a body having two parts of thebody separated by a slit and moving the two parts toward and away fromone another. The cylindrical shell supporting the wrapped fiber isstably mounted at the fixed anchors and can be moved at high frequencywith low forces at the actuator at the edge of the slot. In this way therequired signal can be applied to the fiber at the required amplitudeand frequencies without difficulty and effectively. In thisimplementation the actuator may be mounted to the free edge of thecylinder at the slit, causing the free edge to move under the influenceof the vibrating mass of the actuator. Alternately, the body of theactuator may be fastened to the same support base as the stationarysupports and the portion of the actuator that is displaced may befastened to the free edge, causing vibration without influence ofstiffness of the edge, mass of the actuator, or resonances thereof. Thisvibration stretches and releases tension on the fiber(s), therebymodulating the longitudinal strain.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided an apparatus forverifying operation of an optical fiber monitoring system comprising:

-   -   an optical fiber system including a fiber to be monitored;    -   a common housing mounted at an end of the fiber to be monitored;    -   a monitoring system to detect changes in the optical fiber        caused by an event on the fiber comprising:        -   a source of light arranged to transmit a monitor signal            along the fiber;        -   a transducer arranged to receive the monitor signal after            transmission along the fiber;        -   and a monitoring processor arranged for analyzing the            monitor signal after transmission along the fiber to detect            changes therein caused by the event to be monitored;    -   and a test device for periodically checking proper operation of        the monitoring system comprising:        -   a fiber disturbance actuator to cause disturbance of a            portion of the fiber at a predetermined location thereon            where the disturbance is characteristic of an event to be            detected;        -   and a test processor periodically operating the fiber            disturbance actuator and for detecting a response of the            monitoring processor caused by changes in the monitor signal            caused by said fiber disturbance actuator;    -   wherein at least the monitoring processor and the test processor        are mounted in the common housing.

In one embodiment, the monitoring processor and the test processor canbe parts of a common processor or in an alternative embodiment themonitoring processor and the test processor are separate processorswhere the monitoring processor communicates to the test processor datarelated to the changes in the monitor signal caused by said fiberdisturbance actuator.

In accordance with a further feature of the invention which can be usedwith any of the above features there is provided a method for operationof an optical fiber monitoring system comprising:

-   -   using a detection system to detect changes in an optical fiber        caused by one or more events on the fiber by:        -   transmitting from a source of light at a transmit location a            monitor signal along the optical fiber;        -   receiving the monitor signal after transmission along the            fiber;        -   analyzing the monitor signal after transmission along the            fiber to detect changes therein caused by the event to be            monitored;        -   and generating an alarm in response to said detected changes            which are indicative of an event;    -   generating a waveform which is arranged to trigger an alarm;    -   and periodically optically injecting a test signal corresponding        to said waveform into the fiber to test operation of the        detection system.

In one embodiment the test signal is injected into an added length ofoptical fiber which is connected to the fiber to be monitored through adevice, such as a coupler, splitter, circulator, whereby the test signalis backscattered into the fiber to be monitored. In a monitor systemthat detects Rayleigh backscatter, it is necessary to present anappropriate signal for detection. In this embodiment, an additionallength of optical fiber is added, and that fiber is excited by a laser,causing Rayleigh backscatter to be launched into the monitored fiber andreturn to the monitoring system.

In this arrangement preferably the added length of optical fiber isterminated in a low reflectance manner.

In this arrangement preferably the added length of optical fiber isterminated using a wavelength specific reflection, such as a Fiber BraggGrating, which acts to return just the added test signal.

In one embodiment the test signal is injected from a laser into thefiber at or adjacent a far end of the fiber.

In this arrangement, an amplitude of the test signal can be lowered tocorrespond to a backscattered signal. For example, the test signal canbe reduced in amplitude by in-line attenuators. In this arrangement,angled connectors and low-reflectance optics can be used to control anyback reflection from the detection system.

In one embodiment the laser at the transmit section of the monitoringdevice generating the monitor signal is periodically disabled whilecontinuing to monitor with receiver. This allows the monitor systemreceiver to detect an injected signal in the absence of a monitorsignal. It allows more sophisticated messaging that is not buried inmonitor signal.

In one embodiment the transmit signal is sent during off times that arenormal to a pulsed laser device. This does a similar effect to the oneimmediately above with the exception that is ties the timing to thepulses of the monitor system which translates into a distance location.The monitor system sends a pulse and interrogates the reflections comingback as a function of time which equates to distance. Tying an injectedsignal to that time base shows it at the far end of the fiber.

In one embodiment the test signal is injected from a laser into thefiber at a position spaced from a far end of the fiber and the signal istransmitted along the fiber by reflection from the far end. In thisarrangement preferably a low internal reflectance variable attenuator,optical switch, or both are placed inline with a high reflectancetermination. Preferably the termination is an unterminated or air-gapconnector. For example the fiber is terminated with a reflection such asa reflective deposition on a connector face, such as gold.

In one embodiment the reflection is periodically turned on or off,creating an effective end of line signal that provides many of thefeatures set forth above.

In one embodiment a variable attenuator is modulated such as with a sinewave, causing the end reflection to vary at a pre-determined rate whichadds an additional layer of security as the monitoring device will watchfor that frequency at that precise location.

In one embodiment, as explained herein, the test signal is a waveformwhich is representative of one event of said events. For example, thewaveform is generated by recording an actual event.

In one preferred arrangement, the monitor signal is analyzed by the DASsystem described above which requires very narrow spectrum lasers oftightly controlled wavelength and a laser used to inject the test signalis controlled and arranged to provide a test signal which meets therequirement of the DAS system.

In one preferred arrangement, the analysis system is modified based onthe received test signal caused by said injected signal so as to tunethe analysis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a system of the present inventioninstalled in a fiber to be monitored by inlet and exit isolation fibers.

FIG. 2 is a block diagram showing a system of the present inventioninstalled as a far end connection in a data transmission fiber to bemonitored using inlet and exit isolation fibers.

FIG. 3 is a block diagram showing a system of the present inventioninstalled as a near end connection in a data transmission fiber to bemonitored using inlet and exit isolation fibers.

FIG. 4 is a block diagram showing a system of the present inventioninstalled as a near end implementation using a separate dedicatedmonitoring fiber in a data transmission cable.

FIG. 5 is a block diagram showing a system of the present inventioninstalled as a far end implementation using a separate dedicatedmonitoring fiber in a data transmission cable.

FIGS. 6 and 7 show sample bursts of cyclical disturbances arranged inenvelopes which provide a specified signature of period and frequencywhich allows the monitoring device to identify the applied disturbancesfrom the fiber actuator.

FIG. 8 shows schematically one example of a fiber disturbance actuatorusing a motor for physically moving one part of the fiber relative toanother.

FIGS. 9A and 9B show schematically in side and end view respectively oneexample of a fiber disturbance actuator using an electromagnetic fieldto physically move a fiber portion which is coated with a materialresponsive to the field.

FIG. 10 shows schematically one example of a fiber disturbance actuatorwhich uses a motor to bend one portion of the fiber relative to another.

FIG. 11 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored.

FIG. 12 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is disturbed by an opticalshutter at the far end of the fiber.

FIG. 13 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is generated as a waveformrelated to or corresponding to a waveform detected from an actual event.

FIG. 14 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the actuator which applies the test signal to the fiberis controlled by a communication system along the same fiber which usesa coupler to extract the control signal.

FIG. 15 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the fiber optic system includes two fibers and themethod includes transmitting from at least one source of light at thetransmit location into the first and second ends of each of the firstand second fibers monitor signals so as to travel along the opticalfiber.

FIG. 16 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the fiber is disturbed by a first arrangement forapplying longitudinal forces on the fiber.

FIG. 17 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the fiber is disturbed by a second arrangement forapplying longitudinal forces on the fiber.

FIG. 18 is a side elevational view of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the fiber is disturbed by a third arrangement forapplying longitudinal forces on the fiber.

FIG. 19 is a top plan view of the arrangement of FIG. 18.

FIG. 20 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is applied by opticallyinjecting the signal into the fiber using a first arrangement where thesignal is injected using an optical coupler.

FIG. 21 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is applied by opticallyinjecting the signal into the fiber using a second arrangement where thesignal is directly injected and inline attenuation components are usedto control an amplitude of the required signal.

FIG. 22 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is applied by opticallyinjecting the signal into the fiber using a third arrangement where anoptical shutter is used to control the injected signal.

FIG. 23 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is applied by opticallyinjecting the signal into the fiber using a third arrangement where anoptical shutter, variable attenuator, or combination of the two are usedto control the injected signal.

FIG. 24 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is applied by opticallyinjecting the signal into the fiber using an optical coupler whichcombines the monitor signal with the injected signal. This holds truefor closed loop systems which are non-locating in regard to the locationof the event as well as systems which act to locate the event along thefiber.

DETAILED DESCRIPTION

As shown in FIG. 1 there is provided a fiber 10 including connector 10Awhich provides an input from a monitoring device such as a fiber opticsecurity system. An isolating fiber portion 11 including inlet and exitportions 11A, 11B is connected into the fiber 10 and acts to displacethe main monitored section of the fiber from the monitoring device. Afiber disturbance generator 12 is mounted in the isolating fiber portionand is controlled by a controller 14. This causes an input disturbanceof a predetermined signature and magnitude, as discussed hereinafter, onthe portion of the fiber 10 which is being monitored by the monitoringdevice.

The controller 14, which can be actuated by a switch 15 or by acommunications interface 16, from the monitoring system acts for causingthe fiber disturbance generator to generate the characteristicdisturbance signature with the required magnitude of disturbance.

The communications interface acts to allow the monitor system toinitiate a test. The mechanical switch can be used by a system operatingperson for generating an autonomous test

In FIG. 3 an installation is provided of the fiber disturbance actuatorsystem including the actuator and the isolation fiber portions at thenear end of the fiber is shown. This consists of the system monitor,such as a Distributed Acoustic Sensor interrogator as discussed above.This detects the characteristic disturbance signature introduced by thefiber disturbance generator. As this type of monitoring system is verysensitive to reflections, the isolating fiber isolates the fiberdisturbance generator from the reflections caused by the connection tothe monitor. Also the isolation fiber portions act to allow recoveryfrom any deadzone effect of the monitor. For example, any monitorutilizing optical time domain reflectometry (OTDR) exhibits a “deadzone”at the front panel, causing it to be blind to events immediately afterthe connection. The internal isolating fiber eliminates that issue sincein effect the dead zone is moved into the isolation fiber area.

The output isolating fiber portion 11B, like the input isolating fiberportion 11A, allows for isolation from the front connector. It alsoallows the system to be connected bidirectionally. The output of thisoutput fiber portion is connected to the fiber 10 to be protected. Thecontroller section causes the fiber disturbance generator 12 to generatethe required signature and amplitude for detection.

The optional external switch 15 can be activated to cause the controllerto cause a test to be performed under control of the system operatingpersonnel, that is, without prior instruction from other equipment inthe control system.

FIG. 2 shows a version of the invention which is similar to the FIG. 3with the differentiation that it is intended to be installed at the farend of the fiber, or at any location remote from the near end wherepower is available. The addition of a communication device 17 connectedto a communication medium such as the shown fiber is used to sendinstructions such as a request for test to the controller and a feedbackfrom the monitor system concerning the amplitude of the disturbances.This has the added benefit of testing the far end of the cable or at alocation remote from the near end, which enhances assurance of security.

FIG. 4 illustrates the monitor fiber which intrinsically protects allthe fibers in the cable by utilizing a spare fiber in the cable.

FIG. 5 illustrates an embodiment that utilizes a pair of availablefibers in a cable: one for monitoring and one for communications betweenthe monitoring device and the near end equipment. This is a preferredembodiment where a system can be installed on spare fibers in a cableadding connectivity assurance as well as health tests and periodictesting to the cable monitor.

FIGS. 6 and 7 show typical patterns P1, P2 and P3 of fiber displacementwhere the fiber disturbance actuator generates a series of spacedenvelopes of signal bursts each containing predetermined periodicdisplacements to the fiber. In FIG. 7, the frequency of the bursts inthe pattern P2 is increased relative to that of FIG. 6 and the period ofthe bursts is decreased. It will be appreciated that various patternscan be generated to create a signature pattern to be detected by themonitoring system. Preferably the frequency of the pulses is chosen tobe out of band with ambient disturbances. Preferably the modulationfrequency provides a duty cycle defining the frequency bursts which ischosen to be dissimilar to natural occurrences which can be expectedfrom machine learning so that the signature pattern can be readilydetermined during the test process. This can include repetitive dutycycle such as 50:50. The duty cycle defining the frequency bursts may bea complex keyed code for security. This can be changed periodically,randomly, on a scheduled basis, or triggered by a coded or uncodedmessage or by detectable amplitude changes to the envelope as shown inpattern P3.

In FIG. 7 the last pulses in pattern P3 are shown at reduced amplitudewhich is determined by analysis of the received monitoring signals sothat the amplitude matches a requirement to provide signals which arelarge enough to be identified and not so large that they interfere withthe normal monitoring process by having changes which are beyond thosewhich are expected and are measurable.

When used for near end applications, system can be implemented with onlythe first isolation fiber, which is provided between the monitoringdevice and actuator device.

When used for the far end application, system is implemented with bothinput and output fibers as a method for isolating the actuator signalfrom reflections.

The system with input and output fiber spools can be used either nearend or far end, and is bidirectional in that it can be opticallyconnected in either direction.

It will be appreciated that the type of disturbance used by the systemactuator is selected to match the technology of the monitoring device;thus for example:

In a modal metric detection system, the actuator typically uses adisturbance device which acts to bend or physical move a portion of thefiber.

In an attenuation detection system, the actuator typically uses adisturbance device which acts to bend the fiber.

In an Optical Time Domain Reflectometer detection system, the actuatortypically uses a disturbance device which acts to bend the fiber.

In a Distributed sensing (DAS/DSS/DTS) detection system, the actuatortypically uses a disturbance device which acts to create changes in themonitor signal which can be detected by this type of monitoring system.

In a Distributed Acoustic Sensing (DAS) detection system, the actuatortypically uses a disturbance device which acts to move the fiber in ashaking or vibrating action, or by inducing strain such as by stretchingthe fiber.

In a Strain monitoring system such as strain gauge detection system or aDistributed Strain Sensing system (DSS), the actuator 12 shown in FIG. 1typically uses a disturbance device which acts to stretch or compressthe fiber.

In a Distributed Temperature Sensing (DTS) detection system, theactuator 12 in FIG. 1 typically uses a disturbance device which acts toheat or cool the fiber.

In a Polarization detection system, the actuator typically uses adisturbance device which acts to bend or shake the fiber, ormechanically rotate the state of polarization such as by moving,shaking, or vibrating paddles which introduce birefringence by changingstress on the fiber by way of bending or rotating.

In an Interferometery detection system, the actuator typically uses adisturbance device which acts to bend or shake the fiber.

As an alternative an active area of actuator can contain a fiber Bragggrating, in which the actuator acts to heat, bend, or stretch thegrating.

The communications interface may contain a dual wavelength/single fiberethernet connection. When used with a single fiber monitor, a two fibersolution will protect a cable. When installing optical cables, it iscommon practice to install cables with more fiber than is needed for theimmediate or foreseeable future. As the bulk of the price of aninstallation is labor, and the price difference when upgrading the fibercount is incremental, unused (called “dark”) fiber are often available.As networks most often utilize 2 fibers each, one for transmit and onefor receive, and as fiber count in cables is typically an even number,often a multiple of 6 or 12, there are often pairs of optical fibersthat are available for use. Single fiber monitoring systems, such asDAS, will utilize one fiber in a pair while leaving the other availablefor communication. Single fiber communications standards, such asEthernet, provide full duplex communication over a single fiber bytransmitting one wavelength, such as 1310 nm, in one direction, andanother wavelength such as 1550 nm in the opposite direction. Combiningthe single fiber monitoring with the single fiber communication providesa complete instance of this invention on a pair of fibers. Single fibercommunications solutions can also be achieved by techniques such asdiffering modulation frequencies, states of polarizations, time divisionmultiplexing, and others.

Similarly, the communications might be realized by use of multiplefibers, such as two. Monitoring and communications functions can beperformed over the same fiber pair(s) by use of wavelength divisionmultiplexing, time division multiplexing, or other multiplexing schemes.

Zone based non-locating monitoring methods are often implemented overtwo fibers, or a fiber loop. Use of multiplexing methods such aswavelength division multiplexing, time division multiplexing, or othermultiplexing schemes can be utilized to share the fibers between themonitoring and the communication systems.

CW detecting monitoring devices, such as zone type network or perimeterprotection devices can detect the frequency with a frequency detectingmethod.

This method can be a hardware phase locked loop within the receivercircuitry of the monitoring equipment. In this embodiment, a disturbanceof known frequency is generated while frequency detection equipment suchas a phase locked loop is used for detection, which registers asuccessful test.

This method can be a software phase locked loop. Similar to the hardwarephase locked loop within the receiver circuitry of the monitoringequipment, a detection algorithm is used within the signal processingsoftware. In this embodiment, a disturbance of known frequency isgenerated while frequency detection algorithm emulating a phase lockedloop is used for detection, which registers a successful test.

This method can be bandpass filters. Similar to the hardware phaselocked loop within the receiver circuitry of the monitoring equipment, ahardware or software frequency filter can suppress all frequenciesexcept that of the disturbance generator. In this embodiment, adisturbance of known frequency is generated while detection equipmentdetects the signal passed through the bandpass filter, which registers asuccessful test.

This method can be Fourier Transforms. The spectra of a received signalcan be inspected for the presence of the disturbance frequency. In thisembodiment, a disturbance of known frequency is generated whiledetection equipment detects the signal as a spectral spike of sufficientamplitude, which registers a successful test.

This method can be correlation, including Wavelet Transforms. Thespectra of a received signal can be inspected for the presence of thedisturbance frequency. In this embodiment, a disturbance consisting of apulse, chirp, wavelet, or other finite signal of known composition isgenerated while detection equipment detects the signal and appliescorrelation or Wavelet Transform to detect the presence of thedisturbance, which registers a successful test.

Zone based products may multiplex communication signal and monitorsignal on the same fiber pair by using wavelength division multiplexers.Zone based systems may be configured for single mode or multimode fiber.

A system as described, but used for zone-based non-location determiningsystems, can omit the internal isolations fiber portions as reflectionsand dead zones are not of concern.

For vibration sensing monitoring sensors, the active device disturbingthe fiber within the disturbance generator may be of severaltechnologies, including but not limited to:

Electromagnetic actuators which cause displacement by energizing a coilor other electromagnetic device. This might include attaching a fiber toa moving portion of a voice coil

As Electromagnetic actuators which cause displacement by coating bydepositing or other technique a sensitive material to the fiber andplacing it within the field with no other moving parts.

The fiber disturbance device 12 shown in FIG. 1 can use a Piezo-electricactuator attached to the fiber and causing the vibration

The fiber disturbance device 12 shown in FIG. 1 can use a Hapticactuator such as the rotating motor type

Mechanical such as rotating cam or sawtooth

The fiber disturbance device 12 shown in FIG. 1 can use an arrangementwhere the fiber is displaced by placing it between stators within anelectrostatic field, and varying the field to displace the fiber

Variable optical attenuator: perturbation is a variation in opticalsignal amplitude

Variable polarization controller: perturbation is variation in opticalsignal polarization

Variable optical mode mixing: use a mode mixer to change modal fill ofoptical signal in MMF fiber, causing a perceived perturbation in ourproducts

In the described system, a fiber Bragg grating may be used within thedisturbance generator. When perturbed, detectable wavelength shifts aredetected.

In systems that determine distance or location, the detection signatureshould occur at a predetermined location, representative of theinstallation. This precise location thwarts attempts to spoof the systemby bypassing with a separate fiber. This attempted spoofing might, atthe fiber patch panel or other convenient locations, replace theconnections to the monitored cable with a fiber. Requiring a precisefiber length and event location significantly eliminates that ability.

The disturbance repetition rate, frequency, or combination of these andother parameters may be arranged to be representative of a unique key.

The system may be placed at the beginning, end, or any location alongthe path of the fiber where electrical power is available.

The device is preferably arranged to produce a heartbeat at aconfigurable, identifiable frequency and cadence that the monitoringinterrogator can reliably interpret as a unique event.

When operating in periodic mode the device can be configured with acryptographic key to communicate time based one-time passwords (TOTP)during tests. This can be used to prove the identity of the test deviceto the sensing device. The password can be encoded by the content of thetest signal. The password can be encoded by the timing of the test. Inthis way, devices operating in the above time based one-time passwordmode can be used not just for testing the sensing system, but forproviding evidence that that sensing cable has not been bypassed.Devices operating in periodic mode only without network access may bebattery powered.

The disturbance Generator will have an adjustable magnitude which willallow perturbation to be adjusted to be appropriate to the magnitude ofthe monitor signal. The amplitude must be high enough to be detected,but not so high as to interfere with signal processing. This may becontrolled from any of several locations, including over a network fromthe monitor end of the fiber, or controlled locally such as a localnetwork, manual adjustment, or serial connection.

FIG. 8 shows schematically one example of a fiber disturbance actuatorusing a motor 30 operating a rotating cam 31 for moving one part of thefiber relative to another part which is held fixed by an anchor 32.

FIGS. 9A and 9B show schematically one example of a fiber disturbanceactuator using an electromagnetic field generator 40 to physically movea fiber portion 41 which is coated with a material 42 responsive to thefield.

FIG. 10 shows schematically one example of a fiber disturbance actuatorwhich uses a motor 50 to bend one portion of the fiber relative toanother.

FIG. 11 shows schematically the components herein. Switch 18 representsa mechanical input device which, when actuated, initiates a test or asequence that leads to the performance of a test. This can alsorepresent other types of interfaces such as contact closures and/orother interfaces to allow the test device to send notifications whencertain activities happen in and around the device; i.e. door closuresor equipment cabinet openings. Signatures generated by the test devicecan communicate notifications of inputs at the test device, and reportto the monitoring device.

As set forth above, various disturbance mechanisms are disclosed forcausing a test to be performed using various arrangement for disturbanceof the fiber.

FIG. 1 describes a first embodiment which uses an actuator 12 fordisturbing the fiber by mechanical movement. In FIG. 12 is shown analternative arrangement using many of the same components of FIG. 1where there is disclosed the concept where the disturbance is carriedout by an optical shutter or switch 12A which is actuated to break thepath in the fiber. This provides compliance to Federal regulations forself-test, but rather than perform them directly after the monitordevice, the test is performed at the far end of the fiber, certifyingthat the fiber being monitored is what is expected.

As set forth above, the shutter 12A is operated to provide a disturbanceof the fiber in a recognizable pattern to assure that the expected fiberis being monitored.

The method of testing therefore includes the steps of periodicallychecking proper operation of the optical fiber monitoring system byproviding the optical shutter 12A at the second end of the fiber remotefrom the monitoring system 10A, periodically operating the opticalshutter by the controller 14 under operation from the communicationsystem 16 to temporarily terminate transmission of light along the fiber10 and analyzing the monitor signal at the monitoring system 10A todetect a termination in the monitor signal caused by the operation ofthe optical shutter 12A.

Turning now to FIG. 13 there is shown a further modification where thetesting system is improved by storing complex waveforms and recordings14A in the controller 14 of the test device. These waveforms arerepresentative of an event such as expected intrusions such as a fenceclimb or fence cut. That is in this method a simulation of an actualevent is carried out at event 10C at a location along the fiber whereevents are expected to occur and the effect of that simulated event isdetected by the signal analysis components of the monitoring system 10Aso as to generate a waveform. Different events can be simulated atdifferent locations to create a library of waveforms each representativeof a respective event. Rather than recordings actual events, waveformscan be generated by modelling or simulation to create the library. Orthe library can contain actual events and simulated events so as to beable to carry out a series of tests on the system.

Thus, this system includes the steps of generating a waveform 14A whichis representative of one event of the library of events, at apredetermined location on the fiber periodically operating the actuator12 to cause a disturbance on the fiber which changes the monitor signalin the fiber; where the actuator 12 is operated by the control system 14based on the waveform 14A.

This is valuable for at least 2 reasons:

The test device playing the recording simplifies a portion of the tuningduring the initial installation. A library of intrusions or signals isplayed while the installation personnel adjusted the monitoring devicewithout the need for a second person performing the intrusions. Thisallows for smaller work crews for the calibration process.

The test device can play back a recording of an intrusion as part of theroutine testing of the monitoring system. Rather than just verify thefiber is being monitored, this allows periodic verification that themonitoring system is performing correctly by detecting representativeintrusions. These are not reported as actual intrusions at themonitoring system 10A as the location and timing of the test signal isknown to the test device. This functionality adds a layer of protectionin that, it not only assures the proper fiber is being monitored, italso assures that they detection calibration has not been altered ordesensitized since the set up calibration.

Turning now to FIG. 14 there is shown an arrangement in which thecommunication to the controller 14 of the actuator 12 is provided byadding a coupler 16B or other device within or attached to the testdevice. In this way a portion of the signal fed to an optical detector16C allows the near end monitoring system 10A to send commands encodedin the test laser pulses through the same fiber 10. This eliminates theneed for additional network connectivity to the far end which uses asecond fiber as shown in FIG. 1. Thus, in this arrangement aninstruction signal is communicated along the same fiber 10 to theactuator 12 to effect the operating of the actuator and there isprovided a coupler 16B on the fiber to extract the instruction signal bythe detector 16C from other signals in the fiber to communicate with theactuator 12 through the controller 14.

Turning now to FIG. 15 is shown a pair of monitored fibers, each with amonitor system and with a termination in the same location. In a singlefiber locating system, it is common practice to monitor two fibers,often within the same cable, with 2 channels of equipment, and thesignals propagating in opposing directions: one traverses the cableclockwise, the other counter-clockwise. This is done for cut protection,if the cable is cut or damaged, one channel monitors CW up to the break,the other channel CCW to the break.

This performs a full fiber monitoring scenario even in the instance of afully severed fiber cable. In the implementation shown above, monitoringthe end of the return cable performs the monitoring function on bothcables as though it were a far end implementation. Furthermore, using asingle test device eliminates some cost by two disturbance actuatorsdisturbing both fibers together, or separate actuators for independentcontrol. Other circuit components can also be shared including the powersupply, controller, etc.

A secondary implementation of—FIG. 15 shows a 2-fiber looped backmonitoring system. Rather than open the optical test circuit at the nearend, this acts to break the connection at the far end, assuring thecorrect fiber is being monitored. This holds true for closed loopsystems which are non-locating in regard to the location of the event aswell as systems which act to locate the event along the fiber.

Thus, the system includes a first fiber 101 and a second fiber 102 whereeach of the first and second fibers extends from the transmit locationat the monitoring system 10A to a remote location 103, 104 and returningalong a continuous optical path to the transmit location. Each of thefirst and second fibers defines first and second ends at the transmitlocation 10A thus defining outward 104, 105 and return portions 106, 106of the fiber. In this case the actuator 12 is located at the terminationend of the fibers and arranged to operate on both fibers. The actuatorcan act on one leg or both legs of the fibers.

Adding the disturbance actuator 12 at the termination end of themultiple fiber loops allows functionality as follows:

Additional implementation of the single disturbance actuator design isthe ability to simplify the test device design by the use of multistrandfiber, such as ribbon finer or bifilar fiber, with the multiple fiberspassing through the actuator. The entire test device is the same exceptfor the number of connectors and the internal multi-strand fiber.

For near end implementations, the monitoring device and the test devicecould be built into the same mechanical chassis.

Additionally, in two-fiber closed loop monitoring systems, this canperform the routine test as required by some federal guidelines forassurance of the fiber monitoring system. By either shared actuators orjust shared control electronics, a single rack space device couldmonitor multiple channels, for example matching an industry standard4-channel monitoring device.

Turning now to FIGS. 16 to 19 there are shown arrangements in which anactuator for periodically generating disturbances in the fiber includescomponents generating longitudinal forces in the fiber at one locationalong the length relative to another location along the length so as tocause changes in length of the fiber between the first and secondlocations. That is, as shown in FIG. 17, the disturbances are generatedby a longitudinally moving anchor 60 located between two stationaryanchors 61 and 62. In this arrangement an electromagnetic actuator 12provides movement of the anchor 60 longitudinally of the fiber so thatthe portions of the fiber between the stationary anchors and thefloating support are stretched and released alternately to generatelongitudinal strain in the fiber. This has been found to generate thenecessary changes in fiber structure to introduce the signals.

In FIG. 16 the disturbances are generated by wrapping the fiber around acylindrical support body 65 to form two adjacent lengths 10X and 10Y andcommonly generating the longitudinal forces in the adjacent lengths.That is the support comprises a stationary wrapped anchor 65 aroundwhich the fiber is wrapped and there is provided a longitudinally movinganchor 60 movable towards and away from the wrapped anchor as well asfrom stationary anchor 61.

In FIGS. 18 and 19 is shown another arrangement for stretching the fiberwhere the longitudinal forces in the fiber are generated by wrapping thefiber around a cylindrical shell 70 having two parts 71 and 72 of thebody separated by a slit 73 and moving the two parts relative to oneanother. Thus, the shell 70 is held fixed at anchors 74, 75, 76 and 77and a free edge of the shell is moved radially inwardly and outwardly byan actuator 12. This causes the free edge 78 of the shell to moverelative to the anchor 77 to cause stretching of the portions of thefiber bridging the slot 73.

The cylindrical shell supporting the wrapped fiber is stably mounted atthe fixed anchors and can be moved at high frequency with low forces atthe actuator at the edge of the slot. In this way the required signalcan be applied to the fiber at the required amplitude and frequencieswithout difficulty and effectively. In this implementation the actuatormay be mounted only to the free edge 78 of the cylinder at the slit,causing the free edge to move under the influence of the vibrating massof the actuator 12.

Alternately, the body of the actuator 12 may be fastened to a supportbase connecting the stationary supports for example adjacent the support77 and the portion of the actuator that is displaced is fastened to thefree edge 78, causing vibration without influence of stiffness of theedge, mass of the actuator, or resonances thereof. This vibrationstretches and releases tension on the fiber(s), thereby modulating thelongitudinal strain.

Turning now to FIGS. 20 to 23 there is shown an is a different way tointroduce a signal that performs the test function, but absent aremechanical actuators. This signal can be any of those described aboveincluding envelopes of sine waves as well as complex waveforms.

The concept is to introduce into the far end of the fiber a signal thatis representative of either a test tone or signal or a signalrepresentative of a nefarious event directly as an optical signal. Thereare several ways to accomplish this.

The system uses as the monitoring system, for functionality of thelocating vibration sensor, the arrangement described above as theDistributed Acoustic Sensor (DAS), which acts to measure variations inRayleigh backscattered signal caused by the event to be detected. Thisbackscatter is typically 5 orders of magnitude lower than the incidentoptical power, which requires any externally introduced signal to be ofsimilar magnitude. Additionally, the DAS system is sensitive toreflection such as those caused by poorly mated, faulty, or missingconnectors.

When introducing a test signal therefore, a low reflection must bepresented to the monitoring equipment. As DAS systems utilize verynarrow spectrum lasers of tightly controlled wavelength, great care mustbe taken to select and control the laser.

As shown in FIG. 20 in a first method there is a shown a secondarybackscatter method. Rather than injecting a signal directly into themonitored fiber, an optical transmitter 91 injects the test signal intoa length 92 of optical fiber which is connected to the fiber to bemonitored through a coupler 90 which can also be a splitter, circulator,or other such device. The test signal passes through coupler 90, islaunched into the terminated fiber, and is then backscattered into thefiber 10. The added fiber can be properly terminated in a lowreflectance manner.

As shown in FIG. 21 in a second method there is a shown a directinjection of a test signal from a laser 94 into far end 95 of the fiber10. Angled connectors and low-reflectance optics are used to control anyback reflection back into the monitoring device.

As this system measures Rayleigh backscattering whose signal amplitudeis roughly 5 orders of magnitude below incident power, it is importantto not reflect signals back to the monitoring system. Any device that isput at the far end must be as non-reflective as possible. In a normalinstallation, and at the far end of an installation of the device asdescribed above, the fiber is terminated for a very low reflection. Ahigh reflection disturbs the detection system within the monitor.

Additionally, amplitude should be lowered to correspond to abackscattered signal. This can be achieved by introduction of in-lineattenuators 96.

As shown in FIG. 22 in a third method there is a shown a modification ofthe direct injection method, differentiated by disabling, by an opticalshutter 96 the laser at the transmit section of the monitoring devicewhile continuing to monitor with receiver. Alternatively, timing couldbe such that the transmit signal is sent during off times that arenormal to a pulsed laser device. The expression “pulsed laser” is meantto indicator a laser that caries in amplitude, turns on and off, or onewhose optical path is interrupted as a function of time. This allows themonitor system receiver to detect an injected signal in the absence of amonitor signal, and allows more sophisticated messaging that is notobscured in monitor signal.

That is, as the end of fiber reflection is problematic if not tamed, wecan use this to our advantage as shown in FIG. 23 by controlling it as asignaling mechanism. A low internal reflectance variable attenuator 97,optical switch 98, or both are placed inline with a high reflectancetermination 99. This termination might be a simple unterminated orair-gap connector, or it might be a greater reflection such as areflective deposition on a connector face, such as gold. This reflectioncan be turned on or off, creating an effective end of line signal thatdoes provide both several features as described above.

Additionally, a variable attenuator may be modulated such as with a sinewave, causing the end reflection to vary at a pre-determined rate. Thisadds an additional layer of security as the monitoring device will watchfor that frequency at that precise location.

FIG. 24 is a schematic illustration of the operating components whichform the elements of the invention for attachment to the fiber to bemonitored where the signal in the fiber is applied by opticallyinjecting the signal from a transmitter 16Y into the fiber using anoptical coupler 16X which combines the monitor signal with the injectedsignal. This holds true for closed loop systems which are non-locatingin regard to the location of the event as well as systems which act tolocate the event along the fiber.

In the closed loop system, the monitoring system transmitter injects alight signal into the fiber, which is typically looped back to thereceiver. At the loopback point, an optical coupler enables a testsignal to be superimposed on the monitor signal, simulating thefluctuation in signal representative of an intrusion or otherdisturbance.

1. A method for verifying operation of an optical fiber monitoringsystem: wherein the system comprises a first fiber and a second fiber;each of the first and second fibers extending from a transmit locationto a remote location and returning along a continuous optical path tothe transmit location; each of the first and second fibers definingfirst and second ends at the transmit location thus defining outward andreturn portions of the fiber; the method comprising: using a detectionsystem to detect changes in an optical fiber caused by an event to bemonitored by: transmitting from at least one source of light at thetransmit location into the first and second ends of each of the firstand second fibers monitor signals so as to travel along the opticalfiber; receiving the monitor signals after transmission along thefibers; analyzing the monitor signals after transmission along the fiberto detect changes therein caused by disturbances on the fiber; andperiodically checking proper operation of the optical fiber monitoringsystem by: at a predetermined location on the first and second fibersperiodically operating an actuator to cause a disturbance which changesthe monitor signal in the fiber; analyzing the monitor signal to detectchanges therein caused by disturbance; and in the event that expectedchanges in response to said disturbance are not detected, actuating awarning that the intrusion detection system is not properly operating.2. The method according to claim 1 wherein said disturbances aregenerated by physical manipulation of the fibers.
 3. The methodaccording to claim 1 wherein said disturbances are generated by physicalmanipulation of the first and second fibers by a common actuator.
 4. Themethod according to claim 1 wherein said disturbances are generated onboth the outward and return portions of the first and second fibers. 5.The method according to claim 1 wherein the fiber disturbance actuatorgenerates a predetermined pattern of disturbances.
 6. The methodaccording to claim 1 including analyzing the monitor signal andadjusting an amplitude of disturbance caused by the fiber disturbanceactuator in response thereto.
 7. The method according to claim 1 whereinthe fiber includes an input fiber length for isolation of the fiberdisturbance actuator from the monitoring device.
 8. The method accordingto claim 1 wherein there is provided an output isolating fiber whichallows for isolation from a front connector from the isolation fiber toa main portion of the fiber.
 9. The method according to claim 1 whereinthe signal is analyzed using a monitoring device which is of a type thatdetermines location and wherein the signal is analyzed to monitors for aspecific location and wherein the determination of the location ensuresthat the fiber disturbance actuator is still connected to the monitoredfiber, and that a substitute fiber is not used to bypass the intendedmonitored cable.
 10. The method according to claim 1 wherein theoperation of the fiber disturbance actuator to cause said disturbancesis actuated over a communicating device such as a data network or serialport.
 11. The method according to claim 1 wherein the operation of thefiber disturbance actuator to cause said disturbances is caused by amanual trigger causing a test to initiate where the test can occurimmediately or after a predetermined or random time.
 12. The methodaccording to claim 1 wherein the operation of the fiber disturbanceactuator to cause said disturbances is caused autonomously; where theoperation can be scheduled or random in occurrence.
 13. The methodaccording to claim 1 wherein the fiber disturbance actuator operates tocause disturbances of the fiber by any one or more of bending, shaking,heating, cooling, compressing or stretching the fiber can also be usedas these create changes in the monitor signal which can be detected bythis type of monitoring system.
 14. The method according to claim 1wherein the fiber disturbance actuator operates to cause physicalmovement of the fiber by energizing an electromagnetic device.
 15. Themethod according to claim 1 wherein the fiber disturbance actuatorcomprises an optical shutter at a remote end of the fiber.
 16. Themethod according to claim 1 wherein the fiber disturbance actuator isactuated based on a waveform which is representative of a known event onthe fiber
 17. The method according to claim 1 wherein an instructionsignal is communicated along the fiber to the fiber disturbance actuatorwhich is located at a location on the fiber remote from the commonhousing and wherein there is provided a coupler on the fiber to extractthe instruction signal from other signals in the fiber.
 18. The methodaccording to claim 1 wherein the fiber disturbance actuator is arrangedfor periodically generating disturbances in the fiber by generatinglongitudinal forces in the fiber at one location along the lengthrelative to another location along the length so as to cause changes inlength of the fiber between the first and second locations.